There's no reason the Myst V Ages can't still be implemented into the story that's being told through MOUL. Noloben, Im sure, has many more secrets than what was revealed in Myst V. Since when does an entire Age have to be a single island? There were plenty of other islands out in the distance.
Laki'ahn is another example. So much of that Age was unused in Myst V. The Laki themselves are out there swimming around in the water fully rendered complete with sound effects but you can only see the tips of their fins in the final game. That ever-day eclipse, Im sure, had some puzzle significance in the original plan too that simply got scrapped when the mechanics were altered.
There exists in Noloben in Myst V a collision map over the entirety of the ocean that you cant get to beacuse of an invisible wall. Using a certain program it is possible to bypass the wall and "walk" around in the normal game controls IN the water. This leads me to believe that at one point swimming or perhaps even taking a boat was going to be possible in the Age.
Something else that bothers me about the Watcher's Sanctuary puzzle is the fact that both the giant elaborate door and the orb in the spiral serve no purpose to the puzzle. As we've seen with Ahnonay and Er'cana, the "wait" puzzles seem to indicate the need for another player which leads me to believe that the Watcher's puzzle was intended to require multiple people to solve. This also explains why our version of the sanctuary in MOUL is public
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Fear is the mind-killer